Transmission delay circuits



March 29, 1932. c H FETTER 1,851,090

TRANSMISSION DELAY C IRCUITS Filed June 21. 1927 R, K 5% V' I; Fitter;

Z a F E W WW INVENTOR.

(j A TTORNEY Patented Mar. 29, 1932 PATENT FFICE CHARLES H. FETTER, OF IYIILLBURIT, NEW JERSEY, ASSIGNOR TO AMERICAN TELE- PHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK TRANSMISSION DELAY CIRCUITS Application filed June 21,

This invention relates to delay circuits, and particularly to arrangements of a mechanical nature for introducing delay in the transmission of signals, such as voice frequency signals which may be transmitted over telephone circuits.

In accordance with this invention there is provided a commutator having a plurality of segments to each one of which a condenser is connected or associated in a suitable circuit arrangement, so that all of said condensers may progressively receive an electrical charge from an electrical source, such as a voice frequency signaling source, or an input circuit, and so that said condensers may subsequently surrender their electrical charges in the same progression to a translating or amplifying device, or an output circuit, employed in the transmission of signals to a distant point. Thus, in this invention, time delay will be introduced in the impression of the charges of a plurality of condensers upon a device or output circuitemployed in transmission to a distant point.

One way to introduce time delay in accordance with the principles of the invention is to associate with a commutator a number of brushes of any well known type, one of which is connected to the electrical source or input circuit. and another one of which is connected to the translating or amplifying device or output circuit. To carry out the invention with such apparatus it is necessary either to rotate the commutator about its axis and to thereby progressively charge the condensers associated with the segments of the commutator, or to maintain the commutator stationary in position and to rotate the brushes about it. The brush connected to the input circuit transmits electrical charges to the condensers in progression as a result of the relative rotation of the commutator with respect to that brush, or vice versa, while the brush connected to the output circuit transmits, in the same progression, the electrical charges impressed upon these condensers. A third brush may, if desired, be provided to short-circuit the various condensers after each charge and discharge, thereby preparing these condensers to again receive a simi- 1927. Serial No. 200,420.

lar charge and to subsequently surrender it, and so on.

It becomes apparent that it is one of the objects of this invention to introduce time delay in electrical circuits by charging a plurality of condensers in progression and by discharging these condensers thereafter in the same progression after a predetermined interval of time has elapsed.

It is another object of this invention to introduce time delay of a definite value by suitably spacing brushes associated with the commutator, and further, to change the interval of delay, as desired, by changing the distance between these brushes.

This invention, as well as its further objects and features, will be better understood from the detailed description hereinafter following, when read in connection with the accompanying drawings, in which Figure 1 represents one circuit arrangement embodying the invention, in which a rotatable disk or commutator and stationary brushes are provided, and Fig. 2 represents another embodiment of the invention, in which the disk or commutator is stationary and the brushes rotatable with respect to the disk or commutator.

Referring to Fig. 1 of the drawings, there is shown a microphone S in series relationship with a battery G so that speech variations may be translated into corresponding electrical variations. These electrical variations are then impressed upon an amplifier A of any well known type, preferably of a vacuum tube type, amplifier A amplifying the electrical variations corresponding to the speech impinging upon the microphone S. The amplified currents are then transmitted through a transformer T to a brush B which is connected to ground through a resistance R and the secondary winding of transformer T The brush B is associated with a commutator or rotatable disk K having a plurality of segments, each of which acts like a condenser in its capacity to ground. Some of these condensers are designated by the reference character C. Two other brushes, B and B are similarly associated with the commutator K. As the commutator K revolves about its axis its segments, which act like condensers, make contact with brushes 13 13 and B in the order stated. As each condenser makes contact with brush 5 B a charge is impressed thereon corresponding to a variation in the signals originating in the microphone S. Each condenser subsequently makes contact with brush B which receives the charge and transmits it to an output circuit, as will be described more fully hereinafter. The third brush B shortecirfilament of vacuum tube V, and these elements provide the bias necessary for the proper operation of vacuum tube V as an amplifier. The filament of the vacuum tube V is heated to an electron emitting temperature by the fiow'of current from a'battery G The potential variations between the grid and filament of the vacuum tube V produce corresponding variations in the current flowing between the plate and filament of that vacuum tube, the circuit interconnecting the -:plate and filament including the primary winding of a transformer T and a battery G These amplified current variations are then transmitted throughthe transformer T to an electrical wave filter F which may be of lo-zany well known type, preferably of the type described in the patent to (31A. Campbell, No. 1,227,113. dated May 22. 1917.. The electr cal wave filter F preferably freely transmits currents of frequencies below adefinite -limit, while substantially suppressing cur- "rents of frequencies above that definite limit. Currents of frequencies above the definite limit may be considered distortions, such as harmonics of the signaling currents, and are clearly undesirable. The output of the electrical wave filter F is then transmitted to an output circuit 0, which may terminate at a distant point.

Fig. 2shows another embodiment of the invention in which the commutator K is maintained in-a stationary position. Each ofthe segments of the commutator is connected to a separate condenser C. These condensers are preferably of the same capacity 5 and have one terminal in common. Slip-- rings SR SR and SR rotate about a common axis and obviously at the same speed.

These slip-rings carry brush-holders H H and H respectively, in rotation about the 35 common axis. Brush-holders H H and H hold brushes B B and B respectively. Brush B; causes the condensers C to be progrcssively charged and brush B causes these condensers to be discharged in the same progression. The brush B is employed to shortcircuit thesecondensers in the same progression through a resistance R The apparatus of F ig. 2 is otherwise similar in all respects to the corresponding apparatus in Fig. land its description, therefore, need not be repeated. it

The operation of the apparatus of Fig. 2 will now be briefiy described, it being understood that the general principles of the operation may be applied equally well to the apparatus of Fig. 1. The output of the amplifier A is transmitted through transformer T and through a resistance R slip-ring SR and brush-holder H to brush B Since brushholder H is pinned or otherwise permanently fasten-ed to the slip ring SR brush B will rotate about the common axis at the same speed and progressively make contact with the various condensers C. The entire potential transmitted by the amplifier A through transformer T is not impressed upon each condenser C because of the presence of the resistance R Resistance R decreases each voltage by a definite amount, though these decreased voltages correspond in every respect to the voltages transmitted by the amplifier A. Condensers O retain the presses corresponding voltagesacross the terminals of the resistance R and between the gr d and filament of the vacuum tube V.

Brush B follows brush B brushBg being held by brush-holder H which is similarly pinned or'otherwise fastened to the slip-ring SR Brush B short-circuits each condenser through the resistance Bgtbereby placing each condenser in proper condition to receive an electrical charge and thereafter to surrender it. It will be obvious that the resistance R is of a suitable magnitude to thereby prevent the circuitsassociated with the yacuum tube V from sustaining oscillations. If each condenser were'short-circuited without series resistance the tendency forthe production of sustained oscillations would obviously be very great.

A divisional application which was filed 'on February 8, 1928, hearing Serial No.

252,831, includes claims specificto the ardiameter ence. If it be assumed that the commutator is stationary and that the brushes rotate ten revolutions per second, i. e., 600 revolutions per minute, then each brush would in effect pass 12,000 condensers in a second. In fact,

each condenser would be passed by each brush ten times per second. With such a commutator and with such a brush speed, it will be apparent that frequencies below 6,000 cycles, approximately, would be transmitted, although those frequencies near the 6,000 cycle limit would be somewhat distorted. Yet satisfactory transmission could be conveniently attained over a band of 41,000 or 5,000 cycles. The higher frequencies which become distorted and still other frequencies introduced by the rotation of the brushes about the commutator could be easily removed by an electrical wave filter, particularly a low pass filter, which may be connected as shown in the drawings. If, in a particular arrangement, a set of brushes rotates about a commutator ten times per second, it is obvious that it is possible to attain a maximum delay in transmission of one-tenth of a second. If

the arrangement made fifteen revolutions per of these arrangements might be connected in tandem, or, on the other hand, a greater num ber of segments might be provided on each commutator, as by making these segments narrower or by increasing the diameter of the commutator. Obviously, by changing the brush speed the maximum delay attainable may be correspondingly changed within practical limits. In general, the interval of time delay is decreased as the brush speed is increased, and vice versa.

Each voltage transmitted by the amplifier A to each of the condensers passes through the resistance R If it be assumed that there exist the ideal conditions of no leakage and no inductance, then the voltage impressed across each condenser will be in which E is the voltage output of the amplifier A, B is the magnitude of the resistance in series with the secondary winding of transformer T C is the capacity of each condenser and if is the time during which brush B makes contact with each condenser. In the example given hereinabove for illustrative purpose, the time It would, at a maximum, be one-twelve thousandth of a second. The impression of such a voltage across each condenser brings about its charge at an uneven rate, the charge being very great at first and much slower thereafter. Yet between definite limits the charge on each condenser is almost directly proportional to the voltage impressed thereon. It is necessary to operate between these limits to prevent amplitude distortion. If it be assumed that e=0.1 E, then The values of R and C may then be tabulated as follows:

R C (Microfarads) 790 1 7 ,900 .1 79,000 .01 790,000 .001 7,900,000 .0001 79,000,000 .00001 The values of R and C may be chosen with particular regard to the type of structure employed and the convenient and practical values of its constants. As has already been stated, after each condenser becomes charged through contact with brush B the voltage across each condenser is then impressed upon the grid circuit of the vacuum tube V through contact with brush 13 Thus, the grid circuit of vacuum tube V will receive a fixed percentage of the output of amplifier A, thereby minimizing the possibility of amplitude distortion.

By suitably choosing convenient values for the constants of the type of structure employed, the time delay may be made substantially independent of frequency, amplitude or other characteristic of the electrical circuit within practical limits. The principles underlying this invention may, for example, be applied to a radio secrecy system in which a band of frequencies corresponding to voice frequency signals is subdivided into a plurality of sub-bands, the sub-bands being interchanged in the frequency spectrum to render the unauthorized reception of the signals practically impossible. In such a system it may be desirable to introduce time delay in the transmission of one or more of the subbands at the transmitting station. The re maining sub-bands may be subjected to delay at ithe receiving station for similar time interva 5.

The particular values stated hereinabove are given merely for illustrative purposes and in practice other values may be chosen within the scope of the invention.

While this invention has been pointed out in certain particular arrangements merely for the purpose of illustration, it is to be distinctly understood that the general prinicples of this invention may be applied to other and widely varied organizations Without departing from the spirit of the invention or the scope of the appended claims. a

What is claimed is: 1. A wave transmission system comprising a commutator having a grounded axis, said commutator having a plurality of segments and a plurality of condensers, one condenser being associated with each segment, three brushes located about said commutator, means for revolving the commutator about its grounded axis at a definite speed, an input circuit in which signals originate, one terminal of said input circuit being connected to one of the brushes and the other terminal to ground, and an output circuit to which said signals are to be delivered, one terminal of said output circuit being connected to another of the brushes and the other terminal of the output circuit being connected to ground, the third brush short-circuiting each a condenser as itmakes contact therewith.

2. A transmission delay system including an input circuit from which voice frequency currents flow, an output circuit to which said voice frequency currents are to be transmitted a predetermined interval of time thereafter, rotatable means, and a pair of stationary brushes adjacent to said rotatable means, said brushes being spaced apart bya constant distance corresponding, to the time delay required, one of said brushes being connected to the input circuit, the other brush being 7 connected to the output circuit.

3. A transmission delay system for signals resembling speech currents including a 1'0- tatable element having a plurality of segments, means for impressing varying electrical charges corresponding to said signals 'upon said segments in progresslon, and means for receiving all of the impressed electrical charges in the same progression a common, predetermined interval of time thereafter.

4. A transmission delay system for voice frequency currents including a rotatable element having a plurality of segments, means for impressing varying electrical charges corresponding to signals upon said segments in progression, means for receiving all of the electrical charges impressed upon said segments in the same progression a common, predetermined interval of t1me thereafter, and

I means for progressively grounding all of said segments before electrical chargesare again impressed thereon.

5. A system for delaying the transmission of signals resembling speech currents including a commutator having a plurality of segments, each segment forming a condenser, two

"stationary brushes located a fixed distance apartat the periphery of said commutator, an input circuit including one of said brushes, said signals flowing through said input circuit, and an output circuit including the Lot-her said brushes, said signals being received in said output circuit a predetermined said input circuit, an output circuit including the pther of said brushes, said signals being received in said output circuit a predetermined interval of time thereafter, a third brush also located at the periphery of said commutator, and a circuit including said third brush for discharging the condenser of each segment. 7

In testimony whereof, I have signed my name to this specification this 17th day of June, 1927.

i CHARLES H. FETTER. 

